1. Field of the Invention
This invention relates generally to a method for finding geometric conductor cross-section weak spots, and, more particularly, an adiabatic method for heating the conductor and measuring the electrical resistance of the heated element.
2. Description of the Related Art
In the manufacture of printed circuit boards, whether monolithic or multi-layered, quality depends upon the conductive integrity of the circuit traces as well as the plated-through holes in the circuit boards. The same considerations apply to discrete electrical conductors, or wires, and any other electrically conductive path of which one might conceive. The manufacturing process occasionally results in one or more conductors having areas of reduced cross-section while maintaining conductive continuity. The problem with constrictions or "neckdowns" is that as a circuit board or card is handled, or as it experiences thermal cycles, is powered on and off, or otherwise stressed, the constriction can result in a defect such as an open circuit, often many months or even years after the circuit board is put into operation.
Many attempts have been made to detect and locate such conductor constrictions so that a circuit card can be repaired because in an extreme case such a constriction would likely lead to breakdown at some future time. An example is U.S. Pat. No. 5,172,063 to Munikoti et al., which discloses a method and apparatus for testing for metal integrity by heating the defect until its support, the glass epoxy substrate itself which surrounds a plated-through hole, changes from a solid to a semi-solid state, resulting in a permanent change in conductor resistive value after cool down. Several cycles of heating and cooling are conducted, with resistance measurements being made after each cooling cycle. The measured resistance is compared with an expected resistance value so that a non-expected result indicates that a defect is present in the network of plated-through holes and interconnecting conductors which are included in the conductive path being tested. This patent contemplates the cycle time to be approximately ten minutes.
Several other examples of attempts to determine weak spots in electrical circuitry are reflected in U.S. Pat. Nos. 3,396,335 to Burr et al., 4,213,087 to English et al., 4,743,847 to Frushour, 4,835,466 to Maly et al., and 5,162,742 to Atkins et al. These patents are exemplary of the directions the prior art has taken to accomplish the desired result of locating potentially defective circuit traces and plated-through holes.
Some of the above patents are examples of non-linear current (NLC) testers, which do not heat the conductor under test but appear to employ a skin effect resistivity method. High current pulser testers, another type, generally are destructive in that the purpose is to "blow out" the weak spot so that it can be found and repaired. Optical tests have also been employed, but these have limitations, among them being a high potential for operator error, a requirement for significant operator training, and the inability to spot internal trace defects in multilayer boards. Other limitations of these methods include being significantly time-consuming, and some are limited to testing for shorts only or being otherwise limited in the value of the tests they conduct.